The 3-dimensional structure of the wild type R subunit will provide important information on how the two monomers are relatively oriented to each other, how the protein dimerizies and how the hinge region folds, which is totally disordered in the deletion mutant structure. This dimerization domain also provides a docking site for anchoring the holoenzyme to a variety of scaffolding proteins both in cytoplasm and the nucleus. The crystals of mutant R subunits have defects in their cAMP binding sites. Their structures at high resolution are essential for defining the molecular features of these cAMP binding sites and for understanding the conformational changes which are induced by cAMP binding.